Skip trivia when searching for named exception (#6218)

Closes https://github.com/astral-sh/ruff/issues/6213.

crates/ruff/resources/test/fixtures/pyflakes/F841_3.py

@@ -147,3 +147,10 @@ def f() -> None:
     global CONSTANT
     CONSTANT = 1
     CONSTANT = 2
+
+
+def f() -> None:
+    try:
+        print("hello")
+    except A as e :
+        print("oh no!")

crates/ruff/src/rules/pyflakes/fixes.rs

@@ -115,6 +115,7 @@ pub(crate) fn remove_exception_handler_assignment(
 
     // Lex forwards, to the `:` token.
     let following = SimpleTokenizer::starts_at(bound_exception.range.end(), locator.contents())
+        .skip_trivia()
         .next()
         .context("expected the exception name to be followed by a colon")?;
     debug_assert!(matches!(following.kind, SimpleTokenKind::Colon));

crates/ruff/src/rules/pyflakes/snapshots/ruff__rules__pyflakes__tests__F841_F841_3.py.snap

@@ -584,4 +584,22 @@ F841_3.py:139:17: F841 Local variable `x` is assigned to but never used
     |
     = help: Remove assignment to unused variable `x`
 
+F841_3.py:155:17: F841 [*] Local variable `e` is assigned to but never used
+    |
+153 |     try:
+154 |         print("hello")
+155 |     except A as e :
+    |                 ^ F841
+156 |         print("oh no!")
+    |
+    = help: Remove assignment to unused variable `e`
+
+ℹ Fix
+152 152 | def f() -> None:
+153 153 |     try:
+154 154 |         print("hello")
+155     |-    except A as e :
+    155 |+    except A:
+156 156 |         print("oh no!")
+